1. Field of the Invention
The invention relates to a device and a method for controlling the pressure in at least one inflatable cuff, preferably a finger cuff, of a blood pressure measuring apparatus provided with a plethysmographic sensoring device, where a plethysmographic signal PG and a cuff pressure signal BP are obtained.
2. The Prior Art
The continuous monitoring of blood pressure in an artery in a non-invasive way has for years been of interest to scientists and researchers. As early as in 1942 R. Wagner in Munich presented a mechanical system which was designed to measure arterial pressure of the arteria radialis by means of the so-called “vascular unloading technique”, which is also known as the principle of the relaxed arterial wall (Wagner R. “Methodik und Ergebnisse fortlaufender Blutdruckschreibung am Menschen”, Leipzig, Georg Thieme Verlag, 1942; Wagner R. et al. “Vereinfachtes Verfahren zur fortlaufenden Aufschrift des Blutdruckes beim Menschen”, Zschr. Biol. 112, 1960). The method for non-invasive blood pressure determination presented by J. Penaz in Dresden in 1973 (Digest of the 10th Inter-national Conference on Medical and Biological Engineering 1973 Dresden), also employs the vascular unloading technique. Due to this technique it was possible for the first time to continuously record—albeit only for a short period—the intra-arterial blood pressure with the use of an electro-pneumatic control loop. In this method light is shone through a finger and pressure is applied to the finger via a servo-control system in such a way that the originally pulsating flow registered by the transmitted light is kept constant.
The method in principle is based on the following control loop: a limb or part of the human body containing an artery, such as a finger, carpus or the temple, is shone through by light from a light source. The light which passes through the limb (e.g. finger) or is reflected by a bone contained in the limb or body part (e.g. carpus, temple) is detected by a suitable light detector and provides an inverse measure for the volume of blood in the extremity (plethysmographic signal PG). The more blood there is in the extremity, the more light is absorbed and the smaller is the plethysmographic signal PG. The mean value of PG is suppressed by a difference amplifier, and the resulting signal PG is fed to a controller unit. In the method of Penaz this controller has a proportional-integral-differential (PID) characteristic. The control signal generated by the PID-controller is amplified and added to a constant set-point value (SP) and fed to a servo-or proportional-valve, which generates pressure in a cuff, which in turn acts on the extremity shone through by the light. The control system is such that the plethysmographic signal PG is kept constant over time by means of the pressure applied. When the heart pumps more blood into the extremity during systole and the PG signal exhibits a tendency to decrease, the PID-controller increases its control signal, and pressure in the cuff rises until the surplus blood is pushed out of the extremity and the PG signal reverts to its previous value. Conversely, when the blood flow into the extremity decreases during diastole with the heart in its fill-up phase, which would lead to a rise in the PG signal, the control signal of the PID controller decreases and causes the pressure applied to the finger to drop. Thus the plethysmographic signal is kept constant. By this control system, which keeps the PG signal and thereby the volume of blood in the extremity constant over time, the pressure difference (transmural pressure) between the intra-arterial pressure and the applied external pressure is zero. Thus the externally applied pressure, i.e. the cuff pressure BP, equals the intra-arterial pressure in the extremity. This permits indirect measurement of the blood pressure by means of a pressure sensor or manometer.
The above description of the Penaz principle assumes the control system to be in “closed loop” operation. The system may also operate under “open loop” conditions, with the control signal of the PID controller not added to the set-point value SP. The pressure in the cuff now does not depend on the plethysmographic signal PG and is solely determined by the set-point value SP. According to Penaz, SP corresponds to the mean arterial pressure in the extremity and is characterised by maximal pulsation of the PG value.
This photo-plethysmographic method has been used in a number of further procedures and devices for the measurement of blood pressure. EP 0 537 383 A1 shows an inflatable finger cuff for non-invasive continuous monitoring of blood pressure. The inflatable cylindrical space of the cuff is pneumatically connected to a fluid source. An infrared light source and a detector are positioned on opposite sides of the finger in a rigid cylinder. There is furthermore provided a valve for filling the cylinder with a gas. Electrical leads for the light source and the detector are passed through the cylinder wall. U.S. Pat. Nos. 4,510,940 A and 4,539,997 A also show devices and methods for continuous, non-invasive blood pressure measurement. A fluid-filled cuff, a light source, a light detector and a difference pressure amplifier are provided. Similar devices for blood pressure measurement are known from U.S. Pat. No. 4,406,289 A.
From WO 00/59369, whose subject is a continuous, non-invasive blood pressure measuring apparatus, an improvement of the proportional valve or rather the pressure generating system is known, together with variants of pressure cuffs for diverse extremities.
All known methods and devices—while partly proposing substantial improvements concerning cuff, proportional valve, determination of the set-point SP, etc.—have one thing in common with the original measuring principle of Penaz: a relatively simple control system operated in “closed loop” mode with a controller, e.g. a PID controller. The control system described by Penaz presents a challenge to automatic control engineering. The following independent systems each with specific disturbance variables are part of the control system:                Pressure generation with pressure source (pump) and proportional valve—pump pressure and valve leakage may vary.        Pressure chamber, cuff and pressure transmission to the arterial blood vessel via the tissue of the extremity.        Pulsating fluctuations of blood flow due to the action of the heart—this is the intended disturbance variable, which is to be compensated by the cuff pressure in accordance with Penaz's principle.        If the extremity used is the finger, the arterial blood vessel is a so-called resistance vessel. This means that the diameter of the artery—and thus the blood volume—may be increased (vasodilatation) or decreased (vaso-constriction) by the autonomous nervous system via the smooth muscles of the vessel wall.        Light source and light detection system. Disturbance variables here are manufacturing tolerances of the parts used and above all the influence of ambient light on the plethysmographic signal PG.        Mean value suppression of the PG signal.        Further disturbances due to fluctuations in the parts used, or due to electrical or mechanical influences.        
These factors almost preclude the possibility of continuous blood pressure measurement according to the Penaz principle over a long period of time, even if the set-point SP is optimally determined under open loop operation.
In U.S. Pat. No. 4,510,940 A an effort was made to cope with these disadvantages. A method for long-time blood pressure measurement is described, in which closed loop operation is interrupted periodically and SP is newly determined under open loop operation. This method is a compromise and has the disadvantage that blood pressure fluctuations occurring during the periodic search for optimum SP are not detected.
It is the aim of the present invention to propose, based on the initially described methods and devices, an improved control procedure and a corresponding apparatus for blood pressure measurement implementing the procedure, in which a plethysmographic signal PG and a cuff pressure signal BP are obtained. In particular, long-time indirect measurement of continuous blood pressure is to be guaranteed.
The invention achieves this aim by                a) using the cuff pressure signal BP in a first, inner control loop as control variable and feeding it as a first input signal into a difference amplifier,        b) feeding the plethysmographic signal PG, with mean value PG suppressed, into a controller, preferably a PID controller, in a second, outer control loop, adding a set-point signal SP and generating a target signal SW, which is fed as a second input signal into the difference amplifier, and        c) by using the output signal AS of the difference amplifier to control at least one valve connected to a pressure source, i.e. preferably a proportional valve, which in turn regulates the pressure in the cuff.        
A device for controlling the pressure in at least one inflatable cuff, preferably a finger cuff, of a blood pressure measuring apparatus, which has a plethysmographic sensor device for obtaining a plethysmographic signal PG and a pressure sensor for obtaining a cuff pressure signal BP, is characterised in that two control loops acting on a difference amplifier are provided, where the first, inner control loop uses the cuff pressure signal BP as a first control variable and where the second, outer control loop is provided with a controller, preferably a PID-controller, which generates a target variable SW from the plethysmographic signal PG as a second control variable, and where the output of the difference amplifier controls at least one valve which is connected to a pressure source, i.e. preferably a proportional valve, thereby regulating the pressure in the cuff. The second control loop is provided with a difference amplifier of known design, which subtracts the plethysmographic signal PG from its mean value PG, and with a summation unit (13) adding a set-point signal SP.
The present invention describes a novel control procedure which will permit long-time indirect measurement of continuous blood pressure. The control procedure can be realised either as electronic circuitry or it may be implemented on a computer having program and data-storage capabilities. Peripheral control loops can preferentially be implemented on a computer in program form, while faster, inner control loops containing the drivers for the pressure generation system or for the light-generation and light-detection systems, are preferably realised as electronic circuits. A precise distinction between software and electronic circuitry will not be required in the context of the invention.
The basic principle of the proposed control procedure consists in providing specific control loops, which are preferably concentric, for precisely defined temporal properties and parameters of the whole control system (fast pressure build-up and decrease, compensation of transmural pressure over a single heart cycle, medium-term fluctuations, long-term drifts). Concentric in this case means that the inner control loop is pertinent to a certain temporal property or parameter of the control system and presents idealised conditions for this temporal property to the immediately following outer control loop. This immediately following outer loop may act as an inner loop for yet another outer loop. Preferentially, the inner loops take care of fast control tasks, while the outer loops are responsible for the long-term stability of the overall control system. Furthermore, there may be provided dedicated control loops for certain specific quantities (e.g. cuff pressure, light detection system, mean value suppression etc.), with control parameters optimised for the respective disturbing variable. These control loops need not necessarily be concentric in the sense explained above.